How can I access focal length of active camera, render resolution using animation nodes most directly?
I don't know of a way to directly access the current Scene's properties in Animations nodes but you can circumvent that with custom properties
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Add 2 custom properties to your camera and drive them using your resolution. Example with the resolution X :



Now you can access theses properties in AN using an "Attribute Input" node.

Note : To access the "Camera" part of the Camera "Object", the path is obj.data

So : the focal lens is accessed with data.lens